The Solar System is clearly divided: rocky terrestrial planets close in to the Sun, gaseous Jovian planets farther out, icy Kuiper Belt objects more distant still. However, exoplanetary systems—planets orbiting other stars—commonly violate those divisions. A whole class of exoplanets known as "hot Jupiters" are large planets with orbits smaller than Mercury's, indicating that planet formation may not follow the same rules in all cases.

As described by Joshua A. Carter et al. in Science, a newly discovered system known as Kepler-36 is even stranger. The star hosts two planets with radically different densities in very similar orbits. One planet is roughly 4.5 times more massive than Earth, indicating it is probably a rocky "super Earth," while the second planet is about 8 times more massive than Earth and roughly Neptune-sized, meaning it is likely gaseous. The difference in density is about 8-fold, indicating the two worlds must have formed in different regions of the star system, yet their orbits differ by only 10 percent. In other words, they are in closer proximity than any two planets in the Solar System, but have a larger difference in density. This challenges both naive planetary formation models that demand strict separation in planet types, and naive migration models in which planets form in one place and drift to another due to tidal forces in the protoplanetary disk.

As the name of the system suggests, Kepler-36 is one of the star systems observed by the Kepler observatory. This orbiting telescope locates exoplanets via transits: when a planet briefly eclipses its host star, in much the same way as the famous Venus transit on June 5 and 6, 2012. The amount of light that is blocked is very small in these cases, but it's enough to be measured, even for relatively small worlds. The shape of the light curve—the dip in received light from the star as the planet passes in front of it—reveals the size of the planet, while the time between the eclipses reveals the size of its orbit.

As the authors of the present study pointed out, the initial analysis of the Kepler data overlooked the Kepler-36 system. This is because the automated search algorithm was designed to seek out perfectly regular transits: eclipses that occur at equally spaced intervals. However, the two planets in the system—designated Kepler-36b and Kepler-36c, while the host star is Kepler-36a—orbit closely enough together that they slow and speed each other up as they pass. This makes the times between transits variable, so it was only when researchers adjusted the algorithm to allow for more sophisticated searches that they turned up the planets.

Even with the improved algorithm, Kepler-36b was hard to spot: it only blocks about 1 percent of the light that Kepler-36c does. However, the astronomers were able to identify both as distinct bodies orbiting the same star, partly due to their mutual interaction: when one planet was slow to arrive to its transit, the other was fast, indicating they were pulling on each other gravitationally. (A similar effect occurs in the Solar System, which is how astronomers in the 19th century discovered Neptune: they predicted where it must be based on its interaction with Uranus.)

Upon analyzing the data, the researchers determined that Kepler-36b orbits at a distance of 0.115 astronomical units (AU), while Kepler-36c is 0.128 AU from the host star. (1 AU is the average distance from Earth to the Sun; Mercury orbits at 0.30 AU, so both of these exoplanets are much closer to Kepler-36a than any planet in the Solar System.) Based on the amount of light they block, Kepler-36b is about 1.5 times Earth's radius in size, but Kepler-36c is 3.7 times larger—comparable to Neptune.

The host star is about the same mass as the Sun, but about 1.6 times the Sun's radius. These results are based on the analysis of the star's spectrum and astroseismology: the variations in light due to turbulence in the Kepler-36a atmosphere. This indicates the star is Sun-like, but older, nearing the end of its life cycle. Knowing the mass and size of the star in turn allowed the researchers to measure the masses of the exoplanets: Kepler-36b being between 4.18 and 4.78 times more massive than Earth, and Kepler-36c having between 7.62 and 8.68 times Earth's mass.

With both the masses and radii, the densities of the planets are easily calculated. Kepler-36b is very dense: about 7.5 g/cm3 (7.5 times the density of water), compared to Earth's density of 5.5 g/cm3. Thus, Kepler-36b must be rocky in composition, though it is much larger and denser than any terrestrial planet in the Solar System, so it is not particularly Earth-like. On the other hand, Kepler-36c's density is about 0.89 g/cm3, meaning it would float on water; this density is greater than Saturn's, but less than any other Solar System planet.

However, it's the contrast in density—a factor of 8—compared to the similarity in orbits that makes the Kepler-36 system odd. No other known star system has such a Laurel-and-Hardy combination: an extreme mass density ratio in planets so close to each other. According to current understanding, planets form from a protoplanetary disk of matter surrounding the young star. Friction and tidal forces within the disk can slow the motion of massive planets in the outer regions, causing them to fall inward toward the star. Whether the Kepler-36 system is consistent with such a model remains to be seen; it doesn't necessarily contradict the standard model, but the authors admitted they do not know whether the system fits or not. It is also possible Kepler-36b may have begun as a gaseous planet, but lost much of its mass through bombardment by the stellar wind (particles streaming out from the host star). This means it could have begun life farther out, similarly to Kepler-36c.

Since the star system is older than the Solar System, the conditions under which it formed are long gone. However, as with the discovery of hot Jupiters, Kepler-36 indicates how diverse planetary systems are, and how naive models for planet formation may not be sufficient to explain this diversity.

There's an iphone app, osmos, that in some modes of play, lets you play around with a planet in a simplified solar system. Simplified because the only gravity comes from the central sun. The game's object is to become the biggest planet by controlling your collisions with other objects in a proto solar system. If you hit a smaller object, you absorb it. If you're hit by a larger object, you're absorbed and the round ends.

The game's is useful in getting a sense of why when planets are sweeping out their neighboring regions, they tend to acquire circular orbits. Bottom line is the planet ends up averaging the angular momentum of the objects it acretes so when it runs into slower moving objects, it slows down and conversely if it's chased down by a faster moving object, it speeds up. Planets that start in highly elliptical orbits tend to get nudged sideways by lateral impacts which rounds out their final orbits.

Play the game enough times and you can see variations in starting strategies that can result in elliptical final orbits.

So even though the gravitational physics are highly simplified, the game still retains enough momentum-transfer physics to give you an intuitive sense of how different solar systems can arise.

Here's something that would be interesting. Imagine being in a nearby star system and pointing a telescope at the Sol System.

What would we see? What could we determine about ours, and how accurately? Has anyone ever done a study of something like this?

I've wondered the same thing. Considering that it takes Jupiter 11 years to go around, it would take at least that long to find it's orbit. (up to 22 years if you started looking just after it passed). With planets further out it only gets worse (Neptune is 165 years). With Jupiter it would then take another 11 years to confirm your first measurement and rule out the potential of a second larger, further out, planet that dimmed the star a similar amount.

How does light dimming compare between our planets to an observer 12,000 light years away. For example would Earth and Jupiter be nearly the same?

How does light dimming compare between our planets to an observer 12,000 light years away. For example would Earth and Jupiter be nearly the same?

At interstellar distances, the Sun, Earth, and Jupiter are all basically the same distance away. So, Jupiter blocks essentially as much of the Sun as it would if it were right on top of it, and the same for Earth. So, Jupiter blocks way more sunlight than the Earth does.

Imagine placing a boulder, dime, and half-dollar at 1000 kilometers a few meters apart. Does the relative distance of the dime and half-dollar to the boulder really matter anymore?

(Strictly speaking, Jupiter gets a small advantage for being closer to an observer, farther from the Sun, but it really doesn't matter at all.)

I'm curious about a couple things. Someone else already confirmed my query about only finding closer in planets via this method as we have to wait a long time for the further out planets to make it twice around their sun.

How accurate are these readings? How do they know they aren't seeing two similarly sized planets at twice the distance. In time these would be cleared up I imagine. But if there are large planets in close and at more traditional distances, is it impossible that two similar sized planets could be in a sequence which creates a regular transit of one of them? Especially since you would need quite a few readings over quite a bit of time.

How accurate are these readings? How do they know they aren't seeing two similarly sized planets at twice the distance. In time these would be cleared up I imagine. But if there are large planets in close and at more traditional distances, is it impossible that two similar sized planets could be in a sequence which creates a regular transit of one of them? Especially since you would need quite a few readings over quite a bit of time.

The duration of transit reveals the speed at which the planet is traveling, which gives its distance from the star. The measurements are accurate to the number of digits given.

Since Kepler has shown us that planetary systems exist, basically everywhere, I think it's about time we switch from the term "the Solar System" to 'our solar system".

If for no other reason than to prevent confusion.

The Solar System is named such because it's a system with the star Sol at its center. To call any other star system a Solar System would be confusing.

That would be the case, if not for phrases like "solar radiation", "solar flare'' and" solar wind" none of which have anything to do with our star, other than the fact that it happens to produce them.

At this point using "the Solar System" is as bad as Apple trademarking the "App Store".

Yeah, that's a good point for those terms. I think substituting "astral" for "solar" would be a better solution, i.e "astral radiation," astral flares," and "astral wind."

Honestly, I didn't even think of the two most common uses. You think green energy is a hard sell now? Try getting people to install "astral panels" on their buildings.

I think's it's best if we just stop capitalizing "solar system", as we look to take our very small place in the galaxy. After all, a solar panel isn't going to stop being a solar panel when some other star is shining on it.

Since Kepler has shown us that planetary systems exist, basically everywhere, I think it's about time we switch from the term "the Solar System" to 'our solar system".

If for no other reason than to prevent confusion.

The Solar System is named such because it's a system with the star Sol at its center. To call any other star system a Solar System would be confusing.

That would be the case, if not for phrases like "solar radiation", "solar flare'' and" solar wind" none of which have anything to do with our star, other than the fact that it happens to produce them.

At this point using "the Solar System" is as bad as Apple trademarking the "App Store".

Yeah, that's a good point for those terms. I think substituting "astral" for "solar" would be a better solution, i.e "astral radiation," astral flares," and "astral wind."

No way, too many connotations with astrology. I prefer what was originally said. Either that or use Stellar instead. Stellar systems, stellar winds, stellar radiation, etc.. It can be used for both our star and other stars and is appropriate in both regards while being easy to differentiate with possessive determiners.

Since those two planets must be locked in a resonance, they should have implications. First for resonance theories of planets, like how the Nice model for our own system predicts so many features of it. And then for migrating planets that can be ejected but also caught by such processes, like how the improved Nice model ejecting a fifth giant non-intuitively makes Uranus and Neptune their current sizes.

Alfonse wrote:

Here's something that would be interesting. Imagine being in a nearby star system and pointing a telescope at the Sol System.

What would we see? What could we determine about ours, and how accurately? Has anyone ever done a study of something like this?

- Detection has been described somewhere as I remember it. I think they were decently sure about all planets except Mercury, which is small for transits but far out for wobble detection.

But since then the Kepler sensitivity has been reevaluated for small planets, it overestimated small stars sizes so overestimated the small planets it saw first. Kepler is a prime transit method type case, so it would have been used to estimate the transits.

There is a small chance that they could see all planets or at least all except Mercury, if they lie line-in-sight which transits demand.

- The above would give at least sizes and distances by transits. Distances would give a rough estimate for surface temperatures.

- Atmospheres like the giants H, CO2, CH4 (um, Neptune I think) could in principle be seen. Certainly Earth oxygen if they can get absorption differences out of transits with Earth behind and in front. Depends on precise line-of-sight, so not many if any observers around nearby stars could see it.

- Saturn's rings can in principle be seen when it tilts during transit. A "Saturn on steroids" (tenths of au large rings) have been seen by transit.

- Most or all of our systems moons are too small in relation to their planets to be seen by direct by transit or indirect by planet wobble during transit. Even our own, I suspect.

Note on language: A "star system" is in general a galaxy (or a star cluster).

Planetary systems are what we discuss and the article gets that correct. The Solar system is a specific planetary system pertaining to the star Sun.

Planet systems would be specific planets and their moons, I guess - but I don't have english as primary language. "Planets" since two planets can in principle orbit each other. Earth and Moon is a planet system that in some respects (composition and relative sizes) is more like a planet binary than a planet with moon.

I too had a hard time reading this as I kept thinking 'solar system' generically instead of it being our's alone.

Maybe we need to start using 'Exo' to prefix things outside of our own solar system, as in exosolar system. Or, as others have stated solar system should be generic to all suns that have planetary bodies circling it. Then, add in a pronoun to define who's solar system you speak of (our solar system, the exoplanets solar system).

My problems with "planetary system" is that they tend to be named after their star. As well as the fact that if you removed all of the planets you'd still have a system comprised of the star and all of the associated junk. Do we still call that a planetary system?

My problem with stellar is that, at a planetary scale, there is a difference between the thousands of stars you can see, and the primary star(s) that dominates any given system. Thus the designation, sun.

In the future, I see "solar system" becoming generic for any systems orbiting stars. Particularly after a few generations have been born under a new sun.

Since Kepler has shown us that planetary systems exist, basically everywhere, I think it's about time we switch from the term "the Solar System" to 'our solar system".

If for no other reason than to prevent confusion.

The Solar System is named such because it's a system with the star Sol at its center. To call any other star system a Solar System would be confusing.

That would be the case, if not for phrases like "solar radiation", "solar flare'' and" solar wind" none of which have anything to do with our star, other than the fact that it happens to produce them.

At this point using "the Solar System" is as bad as Apple trademarking the "App Store".

Yeah, that's a good point for those terms. I think substituting "astral" for "solar" would be a better solution, i.e "astral radiation," astral flares," and "astral wind."

Honestly, I didn't even think of the two most common uses. You think green energy is a hard sell now? Try getting people to install "astral panels" on their buildings.

I think's it's best if we just stop capitalizing "solar system", as we look to take our very small place in the galaxy. After all, a solar panel isn't going to stop being a solar panel when some other star is shining on it.

Well it's still a Solar Panel, you ain't getting no energy at night from them other astrals. Until someone installs one on Kepler xc there's no problem with the term.

Aside from that, except if you're a scientist/you're interested in science/or are writing SF books/films/video games, I don't think we need to worry about such vocabulary right away because almost no one in the current population is interested by a planetary system different than the Solar System.

I don't get why all planets don't start out as gas giants. Rock and gas distribution should be relatively similar in all systems, the only large variable being how long the system coalesces before the star is ignited. At the core of every gas giant is rock. it may exist in a liquid state due to pressure, but remove the gasses and it'll cool to a rocky core.

Now, what removes gasses? The solar wind. mercury was assuredly a gas planet, but its proximity blew the gases out to Venus and a bit beyond. Venus is close to a gas giant, in that it is extremely thick atmosphere, but we thin enough (150 miles deep) that we can land on it, and exist for a few hours before we melt. I much rather see Venus as a rocky gas planet. next we have earth, which has a much thinner atmosphere, the majority of which came from comets after the late heavy bombardment. I'd venture any atmosphere had before hand was gone after we took the uppercut that gave is our wobble and moon. Both us and Venus have atmosphere because we both have strong magnetic fields which shield us from solar wind. Next you get out to Mars, which has a very weak magnetic field and as a result a very thin atmosphere.

The planets in the outer solar system experience much weaker solar wind, and can hold their gasses much easier. In fact by now, I'd expect that all planets have as much atmosphere as they can protect, save for mars, whose field is weakening. It will continue to lose atmosphere at a much faster rate because it is geologically dead.

Assumptions:The force of the stellar wind diminishes on the order of 1/r^2.The force of the magnetic field diminishes on the order of 1/r^2 as well, but because the wind only carries momentum, and the shape of the planetary field, it can be defected much more easily.

I don't get why all planets don't start out as gas giants. Rock and gas distribution should be relatively similar in all systems, the only large variable being how long the system coalesces before the star is ignited. At the core of every gas giant is rock. it may exist in a liquid state due to pressure, but remove the gasses and it'll cool to a rocky core.

Now, what removes gasses? The solar wind. mercury was assuredly a gas planet, but its proximity blew the gases out to Venus and a bit beyond. Venus is close to a gas giant, in that it is extremely thick atmosphere, but we thin enough (150 miles deep) that we can land on it, and exist for a few hours before we melt. I much rather see Venus as a rocky gas planet. next we have earth, which has a much thinner atmosphere, the majority of which came from comets after the late heavy bombardment. I'd venture any atmosphere had before hand was gone after we took the uppercut that gave is our wobble and moon. Both us and Venus have atmosphere because we both have strong magnetic fields which shield us from solar wind. Next you get out to Mars, which has a very weak magnetic field and as a result a very thin atmosphere.

The planets in the outer solar system experience much weaker solar wind, and can hold their gasses much easier. In fact by now, I'd expect that all planets have as much atmosphere as they can protect, save for mars, whose field is weakening. It will continue to lose atmosphere at a much faster rate because it is geologically dead.

Assumptions:The force of the stellar wind diminishes on the order of 1/r^2.The force of the magnetic field diminishes on the order of 1/r^2 as well, but because the wind only carries momentum, and the shape of the planetary field, it can be defected much more easily.

According to current understanding, planets form from a protoplanetary disk of matter surrounding the young star.

That's for gas planets. Rocky planets are believed to be 2nd generation formers -- meaning the star goes supernova and the process of photo-disintegration allows for heavy elements to form. (Normally fusion in stars doesn't produce anything heavier than carbon and never anything heavier than iron due to the binding energy requirements of heavier elements no longer contributing to a sustainable exothermic reaction.)

The expelled material from the star is where all the uranium/lead/iron/nickel/silicon etc comes from that make up rocky planets. The inner rocky planets probably came from Sol the first time it went supernova, or possibly snared from the material provided by another nearby star which went supernova.

If the rocky planets came from Sol, then Sol is at least a 2nd gen star. If that's the case, gas has recollected on Sol's original stellar core remnant and reignited so we really have no idea what the core composition is. Probably a ball of carbon.

I don't get why all planets don't start out as gas giants. Rock and gas distribution should be relatively similar in all systems, the only large variable being how long the system coalesces before the star is ignited.

No it's not. After the big bang occurred there was no matter whatsoever. Eventually came a time where the ambient temperature (now Cosmic Microwave Background radiation) cooled to a point where matter could form. That was hydrogen -- or more accurately protons and there were also free electrons that could combine with said protons.

It was still quite hot so some helium combined as well. As time goes on, you have tons of massive clouds of dust called interstellar clouds. These consisted of about 90% hydrogen / 10% helium. Certain spots were more dense than others due to random distribution so they started to clump up. The clumps became more gravitationally dense and attracted even more gas.

Soon you have lots of clumps, some bigger than others. Many of them all spiraling together due to the way angular momentum starts making things spin when their diameter decreases. The big ones become stars; the smaller ones become gas giants. The only difference between the two is the mass. If a gas giant becomes dense enough it will ignite a fusion reaction and then it's called a "star". Also, there are dwarf stars which are gas giants that teeter on the edge between having a sustainable fusion reaction and not.

There is no rocky material to be found. Rocky material does not form by itself. It takes the heat of a star to fuse gas into heavy material like silica (rock). It takes an even hotter event (a supernova) to produce heavier material like the types of metals that make up the earth.

Scorp1us wrote:

At the core of every gas giant is rock. it may exist in a liquid state due to pressure, but remove the gasses and it'll cool to a rocky core.

That's not true, except for rocky material that has collided with the gas giant during the course of it's life.

About terminology of "solar". Just say "solar system" for goodness sake! Dictionary definitions do change over time based on society's perception and common usage. There are so many examples of that in the English language. No need to be so pedantic in using terminology as to confuse the general audience.

I'm sure in 10 years or so, the dictionary definition will catch up with colloquialism. You can be a forerunner in this progression, or you can help hold it back -- your choice.

My problems with "planetary system" is that they tend to be named after their star. As well as the fact that if you removed all of the planets you'd still have a system comprised of the star and all of the associated junk. Do we still call that a planetary system?

Yes. A star is formed out of a "solar nebula" which must form a "protoplanetary disk" ("proplyd") to make a star, whether or not there will be large planets. (There will always be some dust and likely comets.)

I don't get why all planets don't start out as gas giants. Rock and gas distribution should be relatively similar in all systems, the only large variable being how long the system coalesces before the star is ignited. At the core of every gas giant is rock. it may exist in a liquid state due to pressure, but remove the gasses and it'll cool to a rocky core.

- Protoplanetary disks may have a generic structure, planets may have generic formation pathways, but Kepler shows that every system will be an individual.

- Every giant planet which isn't a brown dwarf probably starts out with a rocky core. [Brown dwarfs are believed to be direct collapse objects akin to stars.]

In some cases those cores may be liquefied by high pressure chemistry later. Bonus points! =D

The gas giants - Jupiters, Neptunes- form by core accretion. Still arguable, but most new finds support this. This means runaway growth pulling in gas until the there forms a gap in the disk and the process stops.

- Small planets can't do core accretion. The new result (this week!) that Mars is as wet as Earth-Moon inside means there is probably a generic process for volatiles such as water.

There is one that fits most observations (arguably). Water can adhere to dust that aggregates to form terrestrials inside the snow line even at high temperatures. In our case the snow line was between Mars (dry, ~ 0.05 % water by mass) and the asteroids (wet, ~ 15-20 % water by mass).

This amount of water, ~ 0.05 % by mass, is also just right to be released by volcanism later and form oceans on Earth and early Mars.

- "Yet Earth's atmosphere is two orders of magnitude less dense than that of Venus at the surface. Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere and lithosphere. ... If both of the reservoirs were released to the atmosphere, Earth's atmosphere would be even denser than Venus's atmosphere. The dominant “loss” mechanism of Earth's atmosphere is not escape to space, but sequestration."

- "The dominant non-thermal loss processes for Venus and Mars, two terrestrial planets without magnetic fields, are dissimilar. The dominant non-thermal loss process on Mars is pick-up from solar winds, because the atmosphere is not dense enough to shield itself from the winds during peak solar activity.[2] Venus is somewhat shielded from solar winds by merit of a denser atmosphere, and solar pick-up is not the dominant non-thermal loss process on Venus."

[My bold]

Note that Venus has a dense CO2 atmosphere still, only because of its runaway greenhouse heat. It has lost its water by photodissociation, and then hydrogen loss to space and (probably) oxygen loss to sequestration (oxidation of warm rocks).

lake393 wrote:

Rocky planets are believed to be 2nd generation formers -- meaning the star goes supernova and the process of photo-disintegration allows for heavy elements to form.

You are confusing stellar generations with individual stars. A star that has gone through the main sequence and consumed its fusion fuel can go supernova, but not be "reborn".

So each generation stars is formed fresh from gas clouds. The clouds collapse to a protoplanetary disk, then they collapse to a star in the center. The disk forms planets while the star starts to ignite and disperse the disk. See my first Wikipedia link above to the current facts and theories.

This is enough for planetary formation, which now have been seen in 2nd and 3d gen stars. Star outflows can form interesting dust, for example complex organics from so called carbon stars (large and old enough to fuse carbon), but not planets.

lake393 wrote:

we really have no idea what the core composition is. Probably a ball of carbon.

The Sun isn't large enough for anything but hydrogen fusion, so that is most of its core. Most stars has a "metallicity" (other elements than H and He) of only ~ 1 %, so it is pretty distributed by convection against any gravitational settling.

I guess if I had a point to make myself, which I've made before, it's that we don't speak Latin, so frankly I don't really mind which convention we use for describing solar systems (can I pluralise that? will it upset anyone if I do?) as long we don't defend it with the bogus justification that it must be so because it's good Latin. Calling the sun "Sol" when speaking English sounds more sci-fi than anything else, and even carries (for me) the connotation that we're attributing some kind of god-like or new age spiritual nature to it. While etymologies are genuinely fascinating to me, let's not forget that we're speaking a living language, and it should make sense in its own context, not that of a two thousand year old dead one's. The notion of _a_ sun belonging to a generic system of orbiting planets is not a new one, and while precision is always desirable in science, if it doesn't cause ambiguity then I don't see the problem. There are far more ambiguous terms used regularly in scientific discourse!

I entirely agree with you about context, though, and defining terms is especially important when it's not unambiguously clear.

Here's a fun factoid. The ratio of the distance to the moon to its radius is roughly 221. The ratio of the larger planet's radius to the difference in the two orbits is roughly 81. So anyone on the surface of the smaller world would see something about 3 times the size of our moon every time the planets passed each other.